The present invention relates to an arrangement for cooling of oil in a gearbox in a vehicle via a separate cooling circuit.
The oil in a vehicle's gearbox has to be at a relatively low temperature if the components intended to be lubricated and cooled by the oil are to achieve a long service life. A known way of ensuring that the oil in the gearbox will substantially continuously be at a low enough temperature during operation of the vehicle is to use an air-cooled oil cooler for cooling the oil. However, for it to be able to provide effective cooling of the oil, an air-cooled oil cooler has to be so positioned in the vehicle that air which is at the temperature of the surroundings flows through it. Air-cooled oil coolers are therefore often situated at the front portion of a vehicle at a location in front of, inter alia, the radiator for cooling the coolant of the combustion engine's cooling system. At this location the oil cooler has a forced air flow passing through it which is at the temperature of the surroundings and is provided by a radiator fan during operation of the combustion engine. A disadvantage of such positioning of the oil cooler is that the oil has to be conveyed forward from the gearbox to the front portion of the vehicle. This entails having to provide relatively long oil lines in the vehicle. Moreover, for safety reasons, the presence of oil in such an exposed region as the front portion of a vehicle is to be avoided.
Another known practice is to use a liquid-cooled oil cooler for cooling of oil in a gearbox. Such oil coolers use the existing coolant in the vehicle's ordinary cooling system for cooling the gearbox oil. Coolant-cooled oil coolers may be situated in the vehicle close to the gearbox. In heavy vehicles, the coolant is usually cooled in a radiator situated downstream of a charge air cooler with respect to the intended direction of air flow through these cooling elements. The air is thus warmed by the charge air cooler before it can be used for cooling the coolant in the radiator. This means that coolant is cooled by air which is at a higher temperature than the surroundings. The coolant can therefore not usually have a low enough temperature imparted to it for it to be able to cool the oil in the gearbox in an optimum manner.
U.S. Pat. No. 6,196,168 refers to a system for warming and cooling of oil in a coolant/oil heat exchanger in a vehicle. The oil which is warmed and cooled in the heat exchanger may be gearbox oil. A valve is used for controlling the supply of coolant to the heat exchanger. After starting of the vehicle, the valve leads uncooled coolant to the heat exchanger from a tank. The oil provides in this case a warming of the coolant. In operating situations where the coolant is warm but at below normal operating temperature, the valve leads a mixture of coolant from a low-temperature cooler and said tank to the heat exchanger to cool/warm the oil. During normal operation of the vehicle when the coolant has reached operating temperature, only coolant from the low-temperature cooler is used for cooling the oil. Even if the coolant which has been cooled in a low-temperature cooler is at a lower temperature than the coolant cooled in an ordinary radiator, the coolant is usually still too warm to cool the gearbox oil to an optimum temperature during operation of a vehicle.